The present invention generally relates to optical heads and optical recording and/or reproducing methods, and more particularly to an optical head and an optical recording and/or reproducing method which are suited for carrying out high density recording and/or reproduction of information with respect to an optical disk.
An optical disk unit can be used as a storage unit such as a file system, and is suited for storing programs and large amounts of data. For this reason, there are increasing demands to use an optical disk such as a magneto-optic disk for mass storage units of multi-media systems which are rapidly developing. In order to satisfy such demands, it is desirable to develop an optical head and an optical recording method which are suited for recording information on the optical disk with a high density.
FIG. 1 shows an example of a conventional magneto-optic head. In FIG. 1, a light beam emitted from a semiconductor laser 31 is formed into parallel rays by a collimator lens 32, and thereafter formed into parallel rays having a circular cross section by a true-circular correction prism 33. The parallel rays from the true-circular correction prism 33 are supplied to an objective lens 35 via a beam splitter 34, and converged on a magneto-optic disk 50 by the objective lens 35. The magneto-optic disk 50 includes a transparent substrate 51 and a recording layer 52, and the objective lens 35 converges the parallel rays from the beam splitter 34 on the recording layer 52 of the magneto-optic disk 50.
The reflected light from the magneto-optic disk 50 is formed into parallel rays by the objective lens 35, and reflected by the beam splitter 34. The reflected light from the beam splitter 34 is divided into two beams by a beam splitter 36, one beam being supplied to a focal error and tracking error detection system and the other beam being supplied to a magneto-optic reproduced signal detection system.
The focal error and tracking error detection system includes a condenser lens 37, a cylindrical lens 38, and a 4-element photodetector 39. A focal error signal and a tracking error signal are generated based on outputs of the 4-element photodetector 39 by well known techniques.
The magneto-optic reproduced signal detection system includes a 1/2 wave plate 40, a deflection beam splitter 41, and photodetectors 42 and 43. A magneto-optic reproduced signal is generated based on outputs of the photodetectors 42 and 43.
FIG. 2 shows a relationship of the spatial frequency and the relative change of the amplitude of the reproduced wave for a case where the information is recorded on and reproduced from the magneto-optic disk 50 using the magneto-optic head shown in FIG. 1. The spatial frequency refers to an inverse number of the period of recording marks on the magneto-optic disk 50.
As may be seen from FIG. 2, the spatial frequency becomes high if the recording density on the magneto-optic disk 50 is increased, and the reproduced waveform consequently deteriorates due to the resolution limit of the optical system of the magneto-optic head. In FIG. 2, there is notable deterioration in the reproduced waveform at spatial frequencies exceeding a "reference point".
In order to solve this problem, a so-called "super-resolution" magneto-optic head shown in FIG. 3 has been proposed. In FIG. 3, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. For example, the magneto-optic head shown in FIG. 3 is described in Yamanaka et al., "High Density Recording by Super resolution in an Optical Disk Memory System", Applied Optics, Vol. 29, No. 20, pp. 3046-3051, Jul. 10, 1990.
As shown in FIG. 3, a light blocking band 45 is provided between the true-circular correction prism 33 and the beam splitter 34, and a condenser lens 46 is provided between the 1/2 wave plate 40 and the deflection beam splitter 41. In addition, slits or pinholes 47 and 48 are respectively provided between the deflection beam splitter 41 and the photodetectors 42 and 43. The light blocking band 45 blocks the light at a central portion of the parallel rays in advance to eliminate low-frequency components, by using the fact that the light amplitude distribution of the light spot converged on the magneto-optic disk 50 and the light amplitude distribution of the parallel rays are in a Fourier transform relationship. Accordingly, when the light obtained via the light blocking band 45 and the beam splitter 34 is converged on the magneto-optic disk 50 by the objective lens 35, it is possible to reduce the diameter of the light spot converged on the magneto-optic disk 50 because the light of the low frequency components which causes the spreading does not exist, thereby making it possible to improve the resolution.
As reported in Yamanaka et al. referred above, when the super-resolution is employed, subsidiary maximums are introduced in the light amplitude distribution of the light spot converged on the magneto-optic disk 50. For this reason, it is necessary to take measures to avoid undesirable effects of the subsidiary maximums on the reproduced waveform. Hence, the condenser lens 46 is used to converge the parallel rays on the photodetectors 42 and 43 via the deflection beam splitter 41. In addition, the light components caused by the subsidiary maximums are eliminated by the slits or pinholes 47 and 48.
However, according to the magneto-optic head shown in FIG. 3, a portion of the light to be irradiated on the magneto-optic disk 50 is blocked by the light blocking band 45. As a result, there was a problem in that the intensity of the light that can be irradiated on the magneto-optic disk 50 when recording the information becomes small.